Atherosclerosis continues to progress structurally in one third of patients on optimal medical therapy and every 30 seconds still, an American will have an acute myocardial infarction, and every minute, one will die from one. These data point to the existence of significant gaps in our understanding of this disease process. The long- term goal of the current line of research is, thus, to identify new disease pathways and treatments in atherosclerosis. Studies in other chronic inflammatory diseases have pointed out a phenomenal reduction in tissue inflammation and injury by specific inhibition of the immunoproteasome. Our own preliminary studies demonstrate the expression of immunoproteasome subunits in the shoulder areas of complicated plaques. Guided by these data, the objective of the current application is to determine the expression pattern and pathophysiological significance of the immunoproteasome in an established murine model of atherosclerosis. Our central hypothesis is that the immunoproteasome contributes to the development of advanced atherosclerotic plaques in mice primarily via its activity in bone marrow-derived inflammatory cells. The rationale for these studies is that identification of the role of the immunoproteasome in atherosclerosis will provide further insight into disease mechanisms and novel therapy options. This proposal pursues three specific aims: 1) Determine a) the expression profile of the immunoproteasome in atherosclerosis in apolipoprotein E-deficient (apoE -/-) mice on a high fat diet and b) the effect and mechanisms of action of pharmacological immunoproteasome inhibition in this atherosclerosis model; 2) Determine the leading cell origin of immunoproteasome activity in atherosclerosis; and 3) Determine the role of the immunoproteasome in interferon gamma cytokine signaling on a) endothelial cell function and b) macrophage plasticity. Under aim 1a, apoE-/- mice will be subjected to a high fat diet and the expression profile of the immunoproteasome over the course of atherosclerosis development will be defined. Under aim 1b, apoE-/- mice will be treated with a specific immunoproteasome inhibitor started with (progression model) or after (regression model) the start of the high fat dit. Under aim 2, chimeric bone marrow transplantation experiments will be performed to assess the influence of genetic deficiencies in immunoproteasome subunits in bone marrow-derived inflammatory cells and specifically in CD+ and CD8+ T-cells on experimental atherosclerosis. Under aim 3, the functional significance of the immunoproteasome for interferon gamma signaling in endothelial cells and macrophages will be studied in vitro and in vivo. This proposal is innovative as it will provide first data in atherosclerosis on an increasingly recognized important regulator of inflammation: the immunoproteasome. This proposal is significant because it has the potential to vertically advance our scientific understanding of the atherosclerotic disease process. Ultimately, this is to reduce the burden of atherosclerosis on the individual and society.